A wide range of methods are known for producing molds and foundry cores. Automated machine molding methods are an economical approach in the area of large batches. Tool-less mold production using so-called rapid prototyping methods or 3D printing methods are an alternative to machine molding methods for small to medium-sized series.
Laser sintering methods that permit tool-less manufacturing were developed based on the Croning Method (DE832937), which is known by the name of its inventor, Johannes Croning. According to this method, a molded part is built in layers from particulate material that is coated with a binder. The binding of the individual loose particles is achieved, for example, by applying energy with the aid of a laser beam (EP 0 711 213).
In practice, the solidification described in the prior art is scarcely reached by means of the polycondensation reaction, since process difficulties occur. An exposure to light that is sufficient for developing the final strength would thus result in a severe shrinkage of the binder casing and this, in turn, would cause a process-incompatible distortion of the present layer. The strengths (green strength) of the molded parts produced in this manner are therefore extremely low during removal of the molded parts—also referred to as unpacking—from the loose sand. This causes problems when unpacking and not infrequently results in damage to the molded parts, rendering them unusable. A method has been described for solving this problem during unpacking by using a soldering lamp and thus additionally solidifying the surface with the aid of a soldering lamp. However, this procedure not only requires a great deal of experience, it is also extremely labor-intensive and time-consuming.
The lack of green strengths is due to excessively small or excessively weak binder bridges. If one wishes to engage in distortion-free production, the binder remains excessively viscous and does not form an adequate bridge.
However, a layering method is described in DE 197 23 892 C1, in which Croning sand is printed with a moderating agent, which causes the activation energy of the printed binder-encased Croning sand to be increased or decreased with respect to the unprinted material, and the sand is then exposed to light with the aid of a thermal radiation source. This is intended to cause only the printed or the unprinted areas to be hardened or bound. The finished molded parts are then removed from the unbound sand. However, it has been determined that suitable moderating agents, such as sulfuric acids, are only poorly suited or not suited at all for being printed with the aid of commercial single drop generators. It has also been determined to be disadvantageous that the unsolidified sand is pre-damaged by the exposure to light to such an extent that it may no longer by fully reused in the method. This not only increases the amount of material used but also the costs and is therefore disadvantageous.
A layering method for producing models is described in US 2005/0003189 A1, in which a thermoplastic particulate material is mixed with a powdered binder and printed in layers with an aqueous solvent. The binder should be easily soluble in the aqueous print medium. The models are subsequently removed from the surrounding powder and possibly dried in an oven during a follow-up process for the purpose of increasing the strength.
A layering method for producing investment-cast original models is described in DE 102 27 224 B4, in which a PMMA particulate material, which is coated with a PVP binder, is printed in layers with a mixture of a solvent and an activator for the purpose of dissolving the binder and activating the binder action.
Either the known methods are tool-dependent processes or the known 3D printing processes achieve green strengths that are too low for the efficient and economically advantageous manufacture of molded parts.